This invention is in the field of very large optical apertures, and in particular in the use of shape-retaining thin film membrane mirrors of optical quality.
Various focusing mirror systems fabricated from a reflective metallized membrane are known in the prior art. Commonly, a differential pressure is established between an enclosed area behind the reflective surface and the ambient pressure to control the contour of the flexible reflective surface. The curvature is controlled by various means, such as: an electropneumatic control system (U.S. Pat. No. 4,179,193); an actuator pushing or pulling on a rear membrane (U.S. Pat. No. 5,016,998): an actuator in physical contact with the rear surface of the membrane (U.S. Pat. No. 4,422,723); a double membrane with a partial vacuum between with a complex edge tensioning system to vary the curvature (U.S. Pat. Nos. 5,680,262 and 5,552,006); and a curvature determined by uniform differential pressure applied to a membrane with a non-uniform radial distribution of thickness or a uniform membrane loaded with a non-uniform differential pressure obtained by localized electrostatic or magnetic pressure (U.S. Pat. No. 4,046,462).
Most of the aforementioned inventions are designed for solar energy concentrators. The surfaces obtained do not approach the optical quality required of an astronomical telescope. The next step toward imaging quality are telescopes having less than xcx9c200 waves of low-spatial-frequency surface error. Telescopes having this level of surface error can be compensated with real time monochromatic holography, requiring spatial light modulator resolutions of no more than 40 lines per mm. This situation should produce a near-diffraction limited image. Very large optical apertures, particularly for space-based systems, could benefit from lightweight, optical quality membrane mirrors. Potential applications include astronomy, imaging and surveillance, and laser beam projection.
Optical quality membrane mirrors have been demonstrated (U.S. Pat. No. 6,113,242) wherein a film is mounted on an optically flat circular ring and stretched over a smaller optically flat circular ring. Pressure or vacuum is separately applied to both the inner disk and the outer annulus to produce a doubly-curved optical quality surface in the inner disk. In the atmosphere, pulling a partial vacuum on the underside of the membrane mirror creates a pressure differential. For use in space, a pressure chamber that is bounded by the mirrored surface and a clear polyimide sheet creates the curvature of the optic. The combination of these two sheets is referred to as a lenticular. The use of a clear sheet or inflatable canopy to maintain the necessary pressure to deform the membrane in space entails several problems. Large strains are required, which can put undue structural requirements on the supporting structure. In addition, the canopy itself can refract the incoming radiation or otherwise interfere with various potential missions, such as laser beam propagation.
It is an object of the present invention to provide an optical quality parabolic membrane mirror for use in space that can be collapsed for launch and resume its shape when deployed in space.
A very large aperture optical membrane mirror for use in space is disclosed along with its method of fabrication. The membrane substrate material is first cast on a spinning, inverted (i.e. concave) mandrel that has the basic desired shaped (i.e., parabolic). Stress-inducing and optically reflective coatings are applied after the substrate has cured. This insures that the membrane mirror keeps its shape after it is detached from the mandrel, and provides a highly reflective surface for the operating wavelength band. A rim structure can be attached to the membrane while still in the mandrel for attachment to other structures.